We have described a recombinant clone of human fetal liver cellular DNA (HLM-2) which contains sequences related to the MMTV genome. Further analysis of HLM-2 and four other MMTV related recombinant clones has shown: (1) the human MMTV gag-pol related sequences are organized in a manner consistent with that of a genetically transmitted proviral genome; (2) the human proviral genome contains a mosaic of sequences characteristic of different general; (3) the HLM-2 proviral genome is representative of a large family (50 to 100 copies per cellular genome) of endogenous retroviral genomes; (4) there are approximately 1,000 copies of the LTR like elements in human cellular DNA; (5) this class of endogenous retroviral genomes has been intimately associated with several primate species during the course of evolution; and (6) expression of HLM-2 proviral related RNA has been detected in a primary breast tumor. Based on our current information, we conclude that the HLM-2 proviral genome does not represent the human analogue of MMTV, but instead, represents a novel class of primate endogenous retroviruses.